Reverse: regret road crumbling (10)
I believe the answer is:
Here is my best explanation:
'reverse' is the definition.
'regret road crumbling' is the subsidiary indication.
'crumbling' indicates anagramming the letters.
'regretroad' is an anagram of 'retrograde'.
(Other definitions for retrograde that I've seen before include "Backward", "Reactionary" and "Moving backwards".)